1. Field of Invention
My switching regulator generally relates to non-constant frequency switching regulator circuitry. More particularly, it concerns a switching circuit that renders the duty ratio of switch ON-time (or OFF-time) to have a linear relationship to an input control voltage.
2. Description of Prior Art
Switching regulators (switching power supplies, switching amplifiers, etc.) utilize fast switching with duty ratio control to achieve efficient power conversion or amplification. Duty ratio control controls the ON-time (or the OFF-time) of the switch relative to the cycle time (cycle time=ON-time+OFF-time). By varying the duty ratio, the average value of the output signal is controlled. This average value, after filtering, is the output of the regulator.
If the cycle time is fixed (constant frequency regulator), a simple circuit can be used to produce a linear relationship between control input signal and duty ratio output. For example, a constant current source charging a capacitor followed by a voltage comparator will produce such a linear relationship if the compare voltage is the control input signal.
Many switching regulators employ variable frequency (cycle time is not fixed) in order to obtain higher efficiency and produce less interference. For example, a flyback converter has higher efficiency if operated in a critically discontinuous mode. Many other types of switching regulators also have higher efficiencies if operated in a discontinuous mode. In variable frequency regulators, the average (filtered) output is no longer linearly related to the ON-time, OFF-time, or cycle time. Simple circuits can not be made to control them. The non-linear relationships usually result in increased sensitivity at one end or both ends of the control range. Such increased sensitivity usually results in oscillation or limit cycling when the control loop is closed.
In McCurdy, U.S. Pat. No. 4,814,684 (Mar. 21, 1989), a second discharge path is turned ON to quickly discharge the timing capacitor. This extends the ON/(ON+OFF) time by reducing the non-linearity of the control relationship, which, in turn, extends the VoutWin ratio. But such a two stage approach gives very limited improvement because for a linear control relationship a square function is desired. The square function comes about from the desire to keep the derivative of [ON/(ON+OFF)] time with respect to a control signal (x), constant. The OFF time is constant, so the above derivative becomes {OFF/[(ON+OFF) squared]} times the derivative of the ON time with respect to x. To satisfy the desired condition, then, the derivative of the ON time with respect to x must be proportional to the (ON time+OFF time) squared.
Smedley, U.S. Pat. No. 5,278,490 (Jan. 11, 1994) describes a cycle by cycle control technique that insures that each cycle has a duty ratio output linearly related to control input signal. This technique still has drawbacks. The integration time constant needs to be short in order to fully charge in one cycle. The higher open loop gain due to the short integration time constant decreases stability when the control loop is closed and increases the likelihood of limit cycling. Also, long cycle times are generated by small capacitor charge currents. These small currents result from subtracting two larger currents (reference and feedback). Noise in either of these two currents results in large pulse jitter. Finally, the cycle by cycle control technique does not address the energy needs of the energy storage capacitor in the output filter. During transients, the output voltage across the energy storage capacitor changes. In order to restore the voltage, the switching regulator must provide energy both to the load and to the energy storage capacitor. The energy supplied to the capacitor is transitory, being required only until the proper output voltage is restored. To supply the needs of the energy storage capacitor, the duty ratio control circuit must be given pre-emphasis during output voltage changes. Furthermore, this pre-emphasis must itself be a function of the output voltage. More pre-emphasis is needed at higher output voltage than at lower output voltage because the energy storage inductor in the switching regulator has less voltage across it and therefore builds current slower. Lack of pre-emphasis leads to poor transient response. This is especially critical for switching amplifiers, which must have fast response. It is also important in switching power supplies because they often supply power to transient loads.
Faulk, U.S. Pat. No. 5,680,036 (Oct. 21, 1997) and Schoft, patent U.S. Pat. No. 4,594,541 (Jun. 10, 1986) describe pre-distortion compensation schemes to linearize the control relationship. Tolerances and drift usually severely limit such compensation techniques.